This invention relates to an electric power steering control system and, particularly, to an electric power steering control system suitable for providing an electrical assisting force for the automobile steering system.
Conventionally, electrical apparatus equipped on automobiles are supplied with power from their batteries, as described in Japanese Patent Unexamined Publication No. 58-16958. The electrically power-assisted equipment (e.g., power steering and power window) which consumes a relatively large amount of power, although it is in a small duty cycle, is bulky and costly because of the use of high-rated semiconductor devices for controlling a large motor current. As an example of such equipment the power steering system will be described on FIG. 1.
When the driver turns the steering wheel 9, its rotation is transmitted through a torque sensor 12 to a pinion 8, which moves a rack 7 to direct wheels 6 and 6'. In response to the output signal of the torque sensor 12 a controller 4 supplies power from a first battery 1 to a motor 10, which drives the pinion 8 through a gear 11 so that the torque required of the steering wheel 9 is reduced.
The motor 10 is not supplied with a current unless the steering wheel 9 is operated. On the other hand, when the driver turns the steering wheel of a stopped vehicle, the motor 10 demands a maximum current. This current, when supplied from the first battery of 12 volts, is as much as 50-70 amperes for automobiles with engine displacements ranging 1800-2000 cc.
The controller 4 controls the motor current using a semiconductor device, which has a capacity determined from the load current. For controlling a maximum current of 60 amperes, the semiconductor device must have a current rating above 60 amperes. The current control circuit is generally operated in a chopping mode, and therefore the semiconductor device must have a peak current rating which is 1.5-2 times higher than maximum d.c. current rating. Such a semiconductor device with a maximum peak current rating of 120 amperes has considerable dimensions, and it hampers the effort to build a compact and inexpensive controller 4.
In addition, the wiring for connecting the motor 4 to the controller 4 also needs to have a large current capacity, and its weight reaches almost 1 kg. The first battery 1, which also supplies power to the headlight and other light sources, has its terminal voltage varied when the steering wheel 9 is operated, and this adversely results in the fluctuation of light intensity.
The semiconductor device creates voltage drop of at least 0.2-0.5 volt, resulting in a significant power loss for a 12-volt power system.
The power steering system is required to provide a maximum torque for not longer than a few minutes duration, and a small motor can withstand the duty. On the other hand, however, a limitation is imposed on the motor current in order to suppress the voltage variation of the first battery. A large torque output can be produced from a limited current source only by using a large motor 10, and therefore a small motor cannot be used even though it meets the thermal capacity.
Another conventional power steering system has a provision of a second generator besides the main utility generator to obtain a power voltage porportional to the engine speed for use as a power source for the power steering motor through the control in accordance with the output of the steering torque sensor. However, even in this power steering system, the problems as described above in connection with the preceding prior art are left unsolved. Moreover, the power steering system of patent publication No-61-125964 is not designed to operate in high-voltage motor drive and therefore it fails to make the drive components compact. In addition, the power steering system is active only when the engine is running, and it does not assist the driver who is engaged in a desperate struggle in a stalled vehicle to get out of the mud. The second generator varies its output depending on the engine speed, resulting in wide variations in the motor current even after using the current chopping control. Particularly, the system merely provides a deficient assisting force when the engine is idling, and does not provide any assistance once the engine is turned off.